25th annual Electrical Engineering Safety Seminar 2015

1. Phillip Wall - MIEAust
Phil.Wall@powerearth.com.au
Earthing Systems in Mining Operations
– managing transfer voltage hazards
25th Electrical Engineering Safety Seminar - Nov 2015

2. Segregate and contain?
OR
Bond all systems?
Earthing Systems in Mines
A high Earth Potential Rise (EPR)
needs to be adequately managed
Should we:
Slide 1

3. Earthing Systems in Mines
Managing transfer
voltage hazards

4. Fault
Current
Impedance
to Ground
Buried Earth Grid
Voltage =
Earth Potential Rise
(EPR)

5. Voltage =
Earth Potential
Rise
(EPR)
V Touch Voltage
Transfer Hazard V
Step VoltageV

6. Mandatory commonly bonded systems:
Power Systems
AS/NZS 3007 AS 2067
Single combined
earthing systems
Unless:
• Underground mining
• Voltage limits cannot
be achieved

7. Mandatory commonly bonded systems:
Lightning Protection Systems
AS/NZS 1768 & AS/NZS 3007
Single combined earthing systems
Unless:
• Underground mining

8. • Reduce: impedance of the earthing system,
earth fault current or fault clearing time
• Surface insulating layers, grading rings or bonding
concrete reinforcement
• Separation of HV and LV earthing systems
• Combined HV and LV earthing systems
• Isolation
• Protective barriers or signs
• Remove non compliant infrastructure (eg telco pit)
Risk Management Treatment Methods

9. Mitigation methods for Lightning effects
- Minimizing the lightning collection area
- Supplementary bonding underground in
accordance to the principles of
AS/NZS1768
- Insulation between separate earthing
systems
- Reducing earth connection resistances

10. Lightning Protection Systems in Mines
• Surface installations, protection - AS/NZS 1768
• For underground mines, bonding principles in
underground - AS/NZS 1768
Transfer of lightning to other parts of the mine?
• Minimised through separated earthing systems.

11. Boreholes:
• Voltages can be controlled with PVC
insulation. (Dielectric Isolation)
• Stopping the casing short (say 10m
above roof mesh) is a similar
outcome to PVC for current.
ACARP PROJECT FINDINGS Project C22003
published 1/6/2015
‘Investigation of the Potential Lightning Impacts on Underground Coal Mines’
Source: ‘Investigation of the Potential Lightning Impacts on
Underground Coal Mines’ by Prof. David Cliff

12. ACARP PROJECT FINDINGS
Boreholes (cont):
• Attenuation ↑ lower soil resistivity
deeper the mine
• Impulsive volts ~ tens of kV
on roof mesh
• Gaps in roof mesh - reduced currents
Project C22003
published 1/6/2015

13. Direct strike to surface transformers feeding U/G parts
ACARP PROJECT FINDINGS Project C22003
published 1/6/2015
• Large Voltages (~MV) transferred
by cable screens to U/G parts
• Potential difference in U/G
soil → cable screen armour was
found to be very high.
• Voltages relatively independent of
mine depth, frequency and length
of power cable

14. • Smaller than direct strike
BUT still significant
• The potentials in U/G Parts↑significantly
for increasing cable lengths for higher freq.
ACARP PROJECT FINDINGS Project C22003
published 1/6/2015
Indirect strike to a cable due to a
horizontal lightning channel

15. Why would we separate earthing systems?
• Majority of cases safer to commonly bond.
– Improves overall system impedance
– Lower EPRs and touch hazards
– Simplest and easiest configuration to maintain
– Less damage to equipment
Separate or Common Earthing Systems?

16. Why would we separate?

17. A note on separations – two scenarios
1. Separation between power systems
EPR from a power system fault:
Main Substation
mine infrastructure
(or Mine Surface Earth)
What is the source of electrical energy?

18. A note on separations
2. Separation for lightning transfer
surface
underground

19. Separated Power Earthing Systems
When EPR cannot be managed through Common Bonding.
 Sparse networks
Separation comes with a few difficulties:
• Confusion from multiple earthing systems
• Hazards within yard due to different potentials on earths
• Lightning Flashover – correct insulation levels
• Earth switches/switchboard earths maintenance
• Single point bonding / cable damage

20. Common Bonded vs Separated
Q: Do the scales tilt at any point?
A: Yes,
however the optimum arrangement is not always apparent;
Lower touch voltages with more exposure
vs
Higher touch voltages with less exposure
Each system differs and requires detailed assessment.

21. Independent
Earthing System
Impedance Earthed - IT systems:
The earthable point of the power system is either isolated from earth or
earthed through an impedance
Provide for the safe management of voltages during earth faults

22. Impedance Earthed systems
According to AS/NZS 3007:2013
“There is potential benefit for electrical
supplies entering underground being
impedance earthed systems”
Readily controls touch
and transfer potential

23. Typical Earthing systems
… and how they go together:
Network Earth
Mine Surface Earth (MSE)
Mine Underground Earth (MUE)
Lightning Earth

24. Network Earth
… is the Earthing system associated with the incoming supply.
Irrespective of earth connection
to the upstream Network
substation, the earth fault
belongs to network.

25. Mine Earth

26. Q: What is a Lightning Earth?
A: An earth termination intended
to discharge lightning currents into
the general mass of the earth.
Lightning Earth and transfer effects

27. What happens when the structure
is used as a downconductor?
Can there be Lightning and
Power System separation?
Lightning Earth and transfer effects

28. Lightning Earth and Mine Earths
Unless suitably protected, all surface structures can be assumed to be
incorporated into a lightning earth.

29. Power Systems Separation
Neutral Earthing Resistor

30. Power Systems Separation
What is
this earth?

31. Where to earth?
Power Systems Separation

32. Insulated and
isolated earth
Power Systems Separation

33. Two earthing systems
within the one yard
Power Systems Separation

34. Insulated
and Isolated
Earth Bar
Power Systems Separation

35. Power Systems Separation
NER connected to
Network Earth

36. How to connect these?
Power Systems Separation

37. Screens bared
back and
insulated Single point
bonded at
Mine Earth
Power Systems Separation

38. Controlled
Area
Single point
bonded at
Mine Earth
Power Systems Separation
Single point
bonded at
Network Earth

39. Feeder Maintenance
Breaker 1 open
Earth Switch 1 closed
Breaker 2 open
Earth Switch 2 closed

40. Lightning transfer effects and U/G mines
According to AS/NZS 3007:
No likelihood of transfer:
 AS/NZS 1768.
If likely transfer effects:
 No direct connection
Lightning earth Mine Undergroud Earth (MUE)

41. Where do we provide the separation?
Mine Earths – Surface and Underground?
Screens
bared
back

42. Mine Underground Earth

43. • Transfer mechanisms
- Transition from Surface to UG
- Through different districts in
UG areas
• Areas of interest:
- sealed areas
- working faces
- return airways
U/G Bonding Practices

44. Borehole Mitigations
• Depends on the construction type of borehole
and what services use the borehole
U/G Bonding Practices
- High Voltage feeders
- Gas Drainage /
Dewatering /
Submersible pumps
- Ballast / Concrete /
Stone dust drop holes
- Steel lined air shafts
- Piezometers and
Extensometers
- Tube bundle (caternaries)
and communications

45. Before sealing up longwalls
• Breaks in mesh in gate roads
• Removal or breaking of pipe lengths
and cables
• Removing or treatment of mesh at
seals
• Careful attention to pipe
penetrations through seals
U/G Bonding Practices

46. Mines are being asked to
implement controls to
minimise lightning effects.
Effective earthing systems
such as separated systems
are shown to reduce
energy transfer.
Summary